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Spotted knapweed (Centaurea stoebe) is a non-native invader potentially harmful to Pitcher’s thistle (Cirsium pitcheri), a threatened dune thistle endemic to the Great Lakes region. Previous studies disagree on whether Centaurea has a negative impact on Cirsium populations. Also little is known about the degree of overlap in habitat occupation and flowering phenology between these species. Using data gathered over an 8-year period, the author investigated the long-term impact of Centaurea on Cirsium abundance in Beaver Island, MI dune systems. The author also examined the degree of overlap in habitat occupation and flowering phenology by measuring environmental factors such as soil moisture, soil temperature, amount of bare ground cover, slope, and aspect along with monitoring the flowering phenology of these species. Overall, reductions in Centaurea abundance did not correspond to a change in Cirsium abundance. Differing environmental factors including soil moisture, soil temperature, site location, amount of bare ground, and the distance from shoreline were found to be significantly associated with the presence of Centaurea and Cirsium, suggesting little overlap in habitat occupation. In addition, there was marginal overlap between Centaurea and Cirsium flowering phenology, indicating that pollinator competition is of minimal concern. By identifying the environmental factors associated with Cirsium presence, the work can be used to help reintroduce Cirsium into suitable dune habitat.

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